Sources of increased timing variability following TMS over motor cortex.

Konkle, Verstynen, Diedrichsen, & Ivry

Variability during rhythmic tapping has been attributed to two independent sources (Wing and Kristofferson, 1973): noise in planning processes, including an internal timing system (central variability) and noise in motor implementation processes (peripheral variability). We predicted that transcranial magnetic stimulation (TMS) applied over primary motor cortex would disrupt tapping performance, with the increase in variability restricted to the estimate of peripheral variability. Seven college-aged participants tapped at one of two different intervals (350ms or 550ms) using the index finger of their right hand. While tapping, TMS was applied at a supra-EMG threshold level over the hand area of the left primary motor cortex or over a control area (medial occipito-parietal junction). At both intervals, motor cortex stimulation increased estimates of peripheral variability relative to control stimulation. Estimates of central variability, however, were not significantly influenced by stimulation over the motor cortex. Thus motor cortex stimulation selectively introduces noise into processes involved in motor execution and does not affect an internal timing system. These results are consistent with the hypothesis that the mechanisms for internal timing are independent of the system responsible for production of the movement itself.